HR: 0800h
AN: V21E-0661    [Abstracts]
TI: Magma Degassing With Horizontal Permeable Gas Flow and Transition Between Explosive and Effusive Eruptions
AU: * Ida, Y
EM: yida@sci.u-hyogo.ac.jp
AF: University of Hyogo, 2167 Shosha, Himeji, Hyo 671-2201 Japan
AB: Although degassing is one of the most important factors that control processes of volcanic eruptions, its mechanism and actual feature are poorly understood. For a simple formulation of degassing rate it is assumed in this study that degassing should take place with horizontal gas flow through permeable magma having a constant permeability. Based on this assumption, we make a computer simulation of non-stationary, one-dimensional magma flow and examine how various conditions of the magma chamber and the conduit influence the ascent process toward an eruption, particularly selection between effusive and explosive eruptions. A central problem of the formulation is what drives the permeable gas flow in magma. We propose here that the horizontal gradient of magma ascent velocity from the center to the wall of the conduit should produce gradient of the gas pressure and drives the gas flow. In this treatment the gas pressure is considered to be first enhanced by decompression associated with the magma ascent and then gradually relaxed with viscous magma flow surrounding gas bubbles. This idea gives the degassing rate that is proportional to the ascent velocity and the proportional coefficient, called _gthe degassing constant_h here, is proportional to the permeability and viscosity of the magma. The computer simulation of magma ascent process using the degassing rate obtained above predicts that an eruption should be explosive and effusive if the degassing constant is smaller and greater than a critical value, respectively. According to the simulation an eruption should tend to be more explosive as the pressure of the magma chamber is higher. Furthermore, an eruption should be first relatively explosive and then effusive in its course as magma discharges from the chamber and the chamber pressure decreases.
DE: 1749 Volcanology, geochemistry, and petrology
DE: 8400 VOLCANOLOGY
DE: 8425 Effusive volcanism
DE: 8428 Explosive volcanism
DE: 8434 Magma migration and fragmentation
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005